1. Field of the Invention
The present invention relates to an apparatus for positioning a surface of a substrate such as mask, reticle, wafer or glass plate used in a process for manufacturing a semiconductor device or a liquid crystal display device, relative to a predetermined plane.
2. Related Background Art
A surface positioning apparatus of this type has been widely used in proximity gap setting, focusing and leveling, and in many cases, it is built in an apparatus for transferring a pattern on a mask or reticle to a predetermined area on a wafer or plate which is a photo-sensitive substrate, or a so-called exposure apparatus. In a projection exposure apparatus (stepper) which reduces a pattern of a reticle through a projection optical system and sequentially transfers it onto a wafer, a depth of focus of the projection optical system is becoming very small as the miniturization of a circuit pattern progresses. Thus, a displacement of a wafer surface from a virtual reference plane (which substantially coincides with a focusing plane of a projection optical system) along an optical axis of the projection optical system is detected by using an auto-focusing system (AF system) disclosed in
U.S. Pat. No. 4,558,949. Then, so-called focusing to finely drive the wafer along the optical axis of the projection optical system in accordance with the deviation of the wafer surface with respect to the reference plane is carried out. In the AF system, a slit pattern image is obliquely projected to a predetermined point in a shot area on the wafer without using the projection optical system, and a reflected image thereof is photo-electrically detected by a synchronous detection method. It is commonly referred to as a fixed point AF system.
In the fixed point AF system, a deviation between the focusing plane and the wafer surface is not directly detected. Thus, if the virtual reference plane described above shifts from the focusing plane of the projection optical system by a drift or the focusing plane shifts by a change in a focusing characteristic of the projection optical system due to the absorption of the exposure light, the shift appears as a residual focus offset when the pattern is projection-exposed to the wafer a method for reducing the residual focus offset is disclosed in U.S. Pat. Nos. 4,650,983, 4,629,313 and 4,952,815.
In the U.S. Pat. No. 4,650,983, a reference pattern is formed on a stage (holder) on which a wafer is mounted, and the reference pattern is reverse-projected to a particular pattern of a reticle through the projection optical system. A level of the stage is adjusted such that a contrast of an image of the reference pattern formed on the particular pattern is maximized. Then, the AF system is calibrated such that the plane on which the reference pattern is formed is detected as best focus plane. In the U.S. Pat. No. 4,629,313, the reference pattern on the stage is a slit-like photo-sensor, and a contrast of a pattern image formed when the slit pattern on the reticle is projected by the projection optical system is detected to identify the best focus plane. In the U.S. Pat. No. 4,952,815, a slit-like light emitting mark is provided on the stage, and an image of the light emitting mark is reverse-projected to a particular mark on the reticle. The stage is driven in X and Y directions and a light transmitted upward from the reticle when the particular mark is scanned by the light emitting mark is photo-electrically detected to detect the best focus plane.
An auto-focusing mechanism including a fixed point AF system and a Z-stage which finely drive a wafer along an optical axis of a projection optical system, and a leveling mechanism for keeping a focusing plane of the projection optical system in parallel to the wafer surface are built in the projection exposure apparatus as a surface positioning apparatus. The leveling mechanism comprises an auto-leveling system (AL system) for detecting an inclination of the wafer surface with respect to the focusing plane of the projection optical system as disclosed in U.S. Pat. No. 4,558,949 and a leveling stage for tilting the wafer in a desired direction.
FIG. 11A shows a main portion of a prior art projection exposure apparatus having a leveling apparatus. In FIG. 11A, a reticle 1 is mounted on a reticle holder 2. An exposure light L1 transmitted through the reticle 1 is directed to a projection optical system 3 which transfers an image of a pattern of the reticle 1 to one shot area on a wafer 4, which is held by a wafer holder (0 table) 5 which is mounted on a leveling stage 6 through three support points. As shown in FIG. 11C, two support points 5a and 5b of the three support points 5a-5c are vertically movable drive points and the remaining one support point 5c is a fixed point. The tilt of the wafer holder 5 may be set as desired by independently adjusting the drive distances of the drive points 5a and 5b by the leveling stage 6.
The leveling stage 6 is mounted on a wafer stage 7 which comprises an XY-stage which is two-dimensionally movable in a plane normal to an optical axis AX of the projection optical system 3, and a Z-stage which is finely movable along the optical axis (Z axis) of the projection optical system 3. The stage unit described above is disclosed, for example, in U.S. Pat. No. 4,770,531. A two-dimensional position (for example, XY coordinates) of the wafer stage 7 is continuously detected by a laser interferometer 8, and the X, Y, Z positioning of the wafer stage 7 and the tilting of the leveling stage 6 are carried out by a drive system 9.
An oblique projection type auto-leveling (AL) system disclosed in U.S. Pat. No. 4,558,949 is provided in FIG. 11A. A detection light L2 from a light source 10 is collimated by an projection objective lens 11, and the collimated light beam is obliquely directed to the wafer 4 with respect to the optical axis AX of the projection optical system 3, to illuminate a substantially entire surface of the shot area. A light reflected by the wafer 4 (collimated light beam) is focused by a focusing objective lens 12 onto a photo-sensing plane of a photo-sensor 13. In the AL system, when the tilt of the wafer 4 changes, a focusing position (center of gravity of light intensity distribution) on the photo-sensiting plane of the photo-sensor 13 also changes (shifts). Accordingly, the tilt of the wafer 4 can be detected. As shown in FIG. 11B, the photo-sensor 13 may be a 4-split photo-sensing element.
As shown in FIG. 11C, the photo-sensor 13 is positioned such that the light reflected by a detection area (which corresponds to a shot area) on the wafer 4 is focused to a center of the photo-sensing plane of the photo-sensor 13 when the surface of the wafer 4 (or the reference plate) having a high planarity is parallel to the best focus plane of the projection optical system 3. Thereafter, the tilt of the wafer 4 is adjusted by the leveling stage 6 such that-the reflected light is focused to the center of the photo-sensing plane of the photo-sensor 13 to keep the parallelism between the focus plane of the projection optical system 3 and the surface of the shot area.
While not shown in FIG. 11A, an oblique incidence type auto-focusing system (fixed point AF system) is provided in the projection exposure apparatus. Accordingly, the surface of the wafer 4 (more exactly, a projection point of a slit pattern image) is positioned at substantially same level as the focus plane by driving the wafer stage 7 (Z-stage) in accordance with the detection signal from the fixed point AF system.
When the planarity of the shot areas on the wafer 4 is high, the surface thereof may be set parallel to the focus plane of the projection optical system 3 by the leveling mechanism. However, when the wafer runs through various processes or a plurality of circuit patterns are formed in one shot area, uneveness having a large level difference may be created in the shot area. Due to such unevenness or ununiformity in applying the photoresist, a center of gravity of a light intensity distribution of the reflected light on the photo-sensor (4-split photo-sensing device) 13 may not represent the tilt of the surface of the shot area in the AL system. Namely, even if the leveling mechanism is operated, it may not be possible to set the surface of each shot area parallel to the focus plane of the projection optical system 3.
FIG. 12A shows sawtooth unevenness formed on the wafer 4A. As shown in FIG. 12A, when a detection light L2 is directed to one shot area SA on the wafer 4A, an average plane of the surface of the shot area (for example, a virtual plane defined at a substantially center of a recess and a projection in the shot area) is not kept parallel to the best focus plane P1 of the projection optical system 3 even if the wafer holder 5 is inclined in accordance with the detection signal from the photo-sensor 13. In actual, it is desirable that the average plane of the shot area coincides with the best focus plane P1 as shown in FIG. 12B.
Further, depending on the process (layer), the tilt of the surface of the wafer 4 relative to the focusing plane of the projection optical system 3 when the leveling mechanism is operated may differ from position to position of the wafer 4, that is, from shot area to shot area, as shown in FIGS. 13A and 13B. FIG. 13A shows one shot line seven shot areas) SA.sub.11 -SA.sub.17. FIG. 13B shows tilts after the leveling of surfaces (average planes) P.sub.11 -P.sub.17 of the seven shot areas SA.sub.11 -SA.sub.17 relative to a run plane (drive plane) P2 of the wafer stage. In such a case, it is not possible to make the surface of each shot area coincide to the focusing plane even if a uniform offset is applied after the leveling.
A method for attaining good focusing over the entire shot area, even if the shot area includes unevenness, has recently been developed and is disclosed in U.S. Pat. No. 5,118,957. In this patent, a pinhole image is obliquely projected to each of a plurality of points (for example, five points) in the shot area without using the projection optical system and the reflected images are collectively sensed by a two-dimensional position detection device (CCD). This method is commonly referred to as an oblique incidence type multi-point AF system and it carries out the focus detection and the tilt detection with a high precision. However, in the multi-point AF system, a shift of the surface of the shot area relative to a virtual reference plane (the focusing plane of the projection optical system) is detected at each of the multiple points for each shot area. Then, the average plane of the shot area is determined based on the shifts at the respective points by a minimum square method or an averaging process to calculate the amount of defocus and the tilt relative to the focusing plane of the average plane. Accordingly, a process time for one wafer significantly increases and a throughput decreases.